Interest in metallocene and non-metallocene single-site catalysts (hereinafter all referred to as single-site catalysts) has continued to grow rapidly in the polyolefin industry. These catalysts are more reactive than conventional Ziegler-Natta catalysts, and they produce polymers with improved physical properties. The improved properties include narrow molecular weight distribution, reduced low molecular weight extractables, and good comonomer incorporation, which allows the production of low-density polymers.
Single-site catalysts are typically soluble in the polymerization reaction medium and are therefore advantageous in solution processes. However, for gas-phase, slurry, and bulk monomer processes, it is useful to immobilize the catalyst on an inert carrier or support. Unfortunately, supported catalysts tend to cause reactor fouling and/or sheeting. Reactor fouling results in many serious problems including poor heat transfer, poor particle morphology, and forced reactor shutdown.
To solve these problems, a number of process and catalyst modifications have been disclosed. For example, U.S. Pat. Nos. 4,792,592 and 4,876,320 disclose electrical methods to control reactor static electricity that leads to fouling and sheeting. EP 811,638 teaches addition of antistatic agents to control static buildup. Other additives have also been used to control reactor fouling. See for example U.S. Pat. Nos. 4,885,370, 4,978,722, 5,026,795, 5,037,905, and PCT Intl. Appl. Nos. WO 96/11960 and WO 96/11961.
In particular, WO 96/11960 and WO 96/11961 disclose catalyst systems formed by combining a metallocene, an activator, and a surface modifier applied to a support. Both references teach that the surface modifier must be added to the support during catalyst preparation. Addition of the surface modifier to the reactor during polymerization leads to fouling and a 65 percent loss in catalyst activity. See Example 6 of WO 96/11961. The preferred modifier amount is less than 3.5 percent of the catalyst weight, and the maximum allowable amount is 10 percent. Failure to observe these limits results in increased fouling and substantial reduction of catalyst activity. For instance, Example 6 of WO 96/11960 teaches a significant loss of activity at a modifier concentration of 5 percent.
EP 811,638 teaches addition of an amine antistatic agent to the polymerization reactor to reduce static buildup that can lead to fouling or sheeting. The antistatic agent is added to the reactor in an amount ranging from 1 to 200 ppm based on polymer produced, preferably from 1 to 100 ppm, and most preferably from 1 to 10 ppm (antistatic agent/polymer produced). Higher amounts lead to losses in catalyst activity. Comparative examples 10 and 11 teach that addition of 200 ppm of an ester or 2000 ppm of an ammonium antistatic agent decreases catalyst activity by 50-80%.
In sum, new ways to prevent reactor fouling in olefin polymerizations with single-site catalysts are needed. Particularly valuable processes would use readily available additives that can be fed directly to the reactor. This would prevent additional catalyst preparation expense. Ideally, the additives would increase or have a negligible effect on catalyst activity.